Piezoelectric transducer



July 15, 1947- J. P. ARNDT, JR 2,423,922

PIEzoELscTRIc 'rmmsnucnn Filed Jan. 11, 1945 2 Sheets-Sheet 1 ATTORNEY July 15, 1947 J. P. ARNDT, JR

PIEZOELECTRIC TRANSDUCER Filed Jan. 11. 1945V 2 sheets-sheet 2 :IIU:

I9 w 25 w w P2 #si #i .CS 1/ f I @j y Partnr/AL www INVENTOR. JOHN P ARNDT JR.

AT TOR/vtr Patented `uly l5, 19u47 v 2,423,922.` PmzoELEc'rmo 'rRANsDUcER John P. Arndt, Jr., Euclid, Ohio, assignor to The Brush Development Company, Cleveland, Ohio, a corporation of Ohio Application January 11, 1943, Serial No. 471,966

1s claims. l

This invention relates to piezoelectric transducers and, more particularly, to transducers of the general type constituted by a plurality of sections of crystalline piezoelectrical material that are joined together to form a composite element which either bends, expands or contracts, twists, or otherwise deforms when subjected to electric potentials.

Previous to the invention, `a number of methods had been proposed for joining to each other the pre-electroded major faces of a plurality of crystal sections and for providing a conductive, externally extending lead for the inner electrodes. One method that has achieved great commercial success is explained in the United States patent to A. L. W. Williams, No. 2,106,143.

In accordance with the teaching of the Williams patent, a multiplate flexing element, for example, is fabricated from two plates cut from a homogeneous Rochelle salt crystal, each of which carries upon at least one of its major faces an electrode comprising colloidal graphite. The electroded faces are cemented together with a lead extension, formed of thin metal, clamped between them. The cementing is a manual operation, inasmuch as each assembly must be treated individually, and the labor cost, naturally, is somewhat high.

One successful cement that has been employed is Rochelle salt, melted in its own water of crystallization. However, it is expensive and, furthermore, it is somewhat ydifficult t`o handle.

Accordingly, an object of this invention is to provide an improved method of manufacturing a piezoelectric transducer of the multiplate type, whereby the number of manual operations shall be minimized and the labor cost, accordingly, diminished.

Another object is to provide a method of manufacturing transducers, of the type described, whereby a more uniform product shall be obtained.

Another object is to provide a method of the type described that may be practiced through the utilization of automatic machinery.

Another object is to provide an improved method of fabricating a piezoelectric transducer, of the type described, whereby an electroded crystal section may be provided with a conductive lead extension and, simultaneously, be joined to a similar section at a single operation.

Another object is to provide a method for manufacturing multiplate transducers from Rochelle salt sections or the like.

A still further object of the invention is to provide a method for manufacturing piezoelectric transducers from crystalline materials having dielectric constants lower than that of Rochelle salt.

Briefly, in accordance with the invention, instead of employing cement for the purpose of joining together the pre-electroded faces of a plurality of crystal sections, the said faces are caused to adhere to the opposite surfaces of a ribbon or strip of metallic foil or the like, through the action of heat and pressure upon a coating of material such as wax or the like, or a synthetic plastic, with which the ribbon has been supplied. The coating may be applied mechanically to the ribbon, thus obviating the manual step of applying cement to the electroded faces of the sections. Furthermore, the application of heat and pressure to the ribbon may be accomplished by automatic or semi-automatic machinery and continuous production can thus be obtained.

Conductive contact between the surface electrodes of the sections and the ribbon may be assured by incorporating finely granulated conductive material, such as silver or the like, into the surface coatings of the ribbon or by Toughening the surface of the ribbon as by providing it with integral struck-out projections or teeth.

The heating may be accomplished inductively or by passing an electric current, either alternating or unidirectional, directly through the foil from an appropriate source.

The novel features considered characteristic yof the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will be fully apparent from the following description of certain specific embodiment-s thereof, when read in connection with the accompanying drawings, wherein:

Figure 1 is a view, partly perspective and partly diagrammatic, exemplifying the method of the invention.

Figure 2 is a plan view of a strip of metal foil, or ribbon, having surface coatings of a material that softens under the influence of heat, to which nely divided conductive particles have been added;

Figure 3 is a side view, in vertical section, of the ribbon taken along a line corresponding to the line III--III in Figure 2;

Figure 4 is a plan view of a. strip of metal foil provided with integral protuberances adapted to project through the surface coatings for the purananas Adimensionalaccuracy:theyax'etobeconsidered solely as suggestive and explanatory of the method to be followed and not as views of actual transducers or apparatus. Equivalent elements, in all ilgures, are similarly designated.

Refcrringnowtolligure lofthedrawings,one embodiment of the method, shorn of non-essential steps, comprises 'ilrst forming a stack or sandwich, so to speak, from a lower section I of piesoelectric material, a similar upper section 3 and an intermediate strip l of metallic foil or thelike which carries an upper coating-1 and a lower coating (not shown) of material that soltens and becomes "tacky" under the influence of .heat but which is substantially rigid after cooling. The maior faces' of the crystal sections, including the two that are in contact with the surface layersonthe foil,carryelectrodesofcol loidal graphite or the like, previously lPPlied thereto by employing the method disclosed in the Williams patent aforementioned. or they may carry metalliferous electrodes deposited thermionically by cathode disintegration or mechanically by a process such as tbe Shoop spray.

A multiplicity of metallic particles, preferably angular, may be intermingled with the surface coatingsonthefoiLasshowninHgureszand 3, or the foil itself may be roughened. or be furnished with a plurality of integral, struck-out projections II as shown in Figures 4 and 5.

After the stack has been assembled, the temperature of the metal foil is next raised sutilcientlygto cause softening of the surface coatings thereon and pressure, indicated by the arrow in Figure 1, is applied for the purpose of squeezing out any surplus coating material and of causing the metallic particl, or the integral projections carried by the foil, to establish conductive cmnection between Vthe foil and substantially the entire electroded surface oi' each section.

In order to heat the foil, it may be connected to an appropriate potential source I, alternating or unidirectional, through a circuit comprising master switch Il, a rheostat Il, an automatic time switch il and an ammeter I1. The rheostat, ammeter and time switch are for the purpose of enabling limitation of the current to such a value and for such a pre-determined time pe riod that the piezoelectric properties of the crystal sections shall not be iniuriously affected.

`Itisdiillculttogivespeciiicvaluesfox'thecurrent necessary to soften the coating material and causeits adherence to the electroded crystal sections, or to state the time required Those factors can be determined only by experiment. inasmuch as they depend upon the particular coating material employed, upon the dimensions ofthesections anduponthe particularmenelectric material. With Rochelle salt sections, itisbestnottcexceedatemperature fortbefoil at which the piaoelectric sections are inwill eventually become available if not alin ccnildmtial use. l when the mentioned compound is employed the coating material an extremely strong bond the crystal sections is sedoes not appreciably deeven though the transducer is kept in over long periods of time.

bdt moves past a nm location, desig- Ainlligure,anoperatorplacesapluof crystal sections, interleaved with coated foil, in each empty pocket, seeing to it that the Asthcbeitmovesfromthelefttowardthe rishttbeendsofthefoilmaybecausedtoengage ilexiblecontact members orrollers 25, disposed inthepathoi'travelofthepockets,toapply momentarilyapentialtheretofromanappropriatesourcell. Thecooledaembliesmaybe removed thereafter from the belt by another operata-,ortheymaybepermittedtodropoif intoasuitablebinllorthelike.

Hotionofthebeltmaybecontinuousorimermittemandmeans (notshown) maybeemployed formovimthecmtactmemberslintoandout of mi; with the foil-extremities, as they pas, if desired Either alternatively or simultaneously with the heating by conduction the ioilsmaybeheatedinduotivelybydisposinga coiltlotcoppertubingorstransuppliedfrcm a suitable source 3l of high frequency potential, adjacent to the path of movement of the belt, as shown in Figure 7.

For some applications, a transducer constituted by a plurality of sections of crystalline material, having a much lower dielectric constant than' that of Rochelle salt, such as a primary phosphate. gives satisfactory results. In transducers of that type, the pre-electroding of the opposed surfaces may be dispensed with and the lead extensions themselves may serve as electrodes. The presence of a thin layer of wax or the like,l devoid of metallic particles, between the crystal face and the lead extension, is not objectionable.

From the foregoing, it will be obvious that the disclosed method marks a decided advance in the fabrication of piezoelectric transducers of the multiplate type. Because of` the consolidation of several steps, formerly necessary, into a single operation, the new method enables a material reduction in the labor cost and, in addition, the transducers produced thereby have extremely uniform electrical characteristics. j

Although several alternative embodiments of the invention have been illustrated and described, the inventor is fully aware of the fact that many modifications thereof will be at once apparent to those skilled in the art. The invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims. l

What is claimed is:

1. The steps in a method f fabricating a piezoelectric transducer which comprise selecting a section of piezoelectric material a face of which has an electrically conductive layer, applying to said face a conductive lead-extension having a surface coating of material capable of adhering to the conductive layer when subjected to heat and pressure, subjecting said surface to heart and pressure and simultaneously establishing a plurality of electrically conductive paths between said extension and said layer.

2. The steps in a method of fabricating transducer elements from at least two sections of crystalline piezoelectric material each of which has an extended substantially planar face, that comprise placing a layer of electrically conductive material upon each of said extended faces, disposing :between said faces an extended conductive lead-in element each surface of which has a coating of material that softens under the influence of heat and pressure, urging said sections toward each other to exert pressure on said coatings, heating said lead-in element to a temperature sufllciently high to cause softening of said coatings and concurrently establishing a multiplicity of conductive paths between the layers of conductive material and the lead-in element.

3. The method as defined in claim 2, characterized in this: that heating of the lead-in element is accomplished by causing an electric current to flow therein.

4. 'I'he method as defined in claim 2, characterized in this: that heating of the lead-in element is accomplished inductively.

5. 'I'he steps in a method of fabricating transducer elements from at least two sections of crystalline piezoelectric material each of which has an extended substantially planar face, that comprises disposing between said faces an extended foil-like electrode each surface of which has a coating of material that softens under the iniluence of heat, urging said sections toward each other to exert pressure on said electrode, heating said electroded face and the conductor means to i approach'sufflciently close to each other to establish a plurality of electrically conductive paths between said conductor means and the electroded face of said piezoelectric material, and thereafter cooling said electrical conductor means to cause said conductor means to adhere to said piezoelectric material.

7. The method as set forth in claim 1 further characterized in this: that heating of the adhesive material is accomplished by causing an elec-,r` .c

tric current to flow in the conductive lead-extension.

8. The method as set forth in claim 1 further characterized in this: that heating of the adhesive material is accomplished by inductively causing an electric current to flow in the conductive lead-extension. y

9. The method as set forth in claim 1 further characterized in this: that heating of the adhesive material is accomplished by electrically connecting leads to the conductive lead-extension and by means of the leads passing electric current through said conductive lead-extension.

10. The method as set forth in claim 5 further characterized in this: that heating of the coating material is accomplished by causing an electric current to flow in the foil-like electrode.

11. l'I'he method as set forth in claim 5 further characterized in this: that heating of the coating material is accomplished by inductively causing an electric current to i'low in the foil-like electrode.

12. The method as set forth in claim 5 further characterized in this: that heating of the coating material is accomplished by electrically connecting leads to the foil-like electrode and by means of the leads passing electric current through said foil-like electrode.

13. The invention as set forth in claim 6 further characterized in this: that heating of the electrical conductor means is accomplished by causing an electric current to flow in the electrical conductor means.

14. The invention as set forth in claim 6 further characterized inthis: that heating of the electrical conductor means is accomplished by inductively causing an electric current to flow in the electrical conductor means.

15. The invention as set forth in claim.6 further characterized in this: that heating of the electrical conductor means is accomplished by electrically connecting leads to the conductor means and passing electric current through the said conductor means.

16. In a piezo crystalline multiplate assembly, a rst plate, electrode means on a surface of said first plate and in intimate electrical contact therewith, a second plate, electrode means on a surface of said second plate and in intimate electrical contact therewith, and electrically conductive sheet means having adhesive means on each 7 8 surface thereof connecting said nrst and said second plates together with their eiectroded turme m. fam in face-to-fa relationship, md sheet The following references are of record in the means including a plurality of integral eiectriille of this patent: cally conductive portions extending at angles to s the plane of said sheet, said integral portions UNITED STATES PATENTS extending through said adhesive means and mak- Number Name Date ing electrical contact between said sheet and the Re. 20.213 Boyer Dec. 22. 1936 said surface electrode means on said first and 1.995.257 Sawyer Mar. 19, 1935 said second plates oi piezoelectric material. li) 2,324,024 Rell-m July 13. 1943 FOREIGN PATWTS Number Country Date 180,787 Great Britain June 8, 1922 JOHN P. ARNDT, JR. 

